Sony A7siii new camera won't display video file thumbnails or play when selected - mp4 files

If it is set to 4:2:2, try 4:2:0 (some QuickTime versions also insist that H.265 must be tagged as hvc1 instead hev1).

Or use VLC or IINA as a viewer. Which macOS are you using?? Big Sur QuickTime Player might be more forgiving.

QuickTime Player might refuse to play some H.265/HEVC flavors. macOS 11 Big Sur is most forgiving but has the following issues with "Codec ID" and "Chroma subsampling" options:

- H.265/HEVC Codec ID hvc1: OK.

H.265/HEVC Codec ID hev1: "This file contains media which isn't compatible with QuickTime Player". Fix: add '-tag:v hvc1' without re-encoding:

ffmpeg -i input.mp4 -vcodec copy -acodec copy -tag:v hvc1 output.mp4

- Chroma subsampling 4:2:0: OK.

Chroma subsampling 4:2:2: "This file contains media which isn't compatible with QuickTime Player". Opens anyway in Big Sur but fails in Mojave.

- You can check those options with Invisor or MediaInfo. VLC and IINA have no issues with either.